With the spread of computer networks in recent years, encrypted communication arts have become extremely important and essential. In a general encrypted communication method, data to be subject to transmission is encrypted at a transmitter side and transmitted as encrypted text data to a receiver side, and at the receiver side, the transmitted encrypted text data is decrypted to obtain the original data subject to transmission. Thus to perform encrypted communication between the two sides, in addition to providing means for sending and receiving data, a means for executing an encryption process must be provided at the transmitter side and a means for executing a decryption process must be provided at the receiver side.
As methods of executing such an encryption process or a decryption process, methods using IC cards have come to be noted. Since priorly, IC cards have been used in various applications as portable information recording media of extremely high confidentiality and recently, companies that distribute multifunctional IC cards, serving as company staff ID cards at the same time, to their respective company staff have been increasing. Such a portable information recording medium that provides confidentiality is optimal for use in an encryption process or a decryption process. Often in a general encryption method, a secret encryption key or encryption key table (here, these shall be referred to collectively as “encryption key information”) is used as a seed in an encryption algorithm. In an encryption process of converting predetermined plaintext data into encrypted text data, a process using this secret encryption key information is executed. In this case, the secret encryption key information used in the encryption process becomes an essential element for a decryption process of restoring the encrypted text data to plaintext data. That is, the decryption cannot be performed as long as the secret encryption key information is not provided. Thus by storing this secret encryption key information in an IC card, access of the encryption key by an illicit means is made extremely difficult and security of the encrypted text itself can be improved.
Encrypted communication using IC cards can be realized by preparing two IC cards storing the same encryption key information. That is, a transmitter is made to own a first IC card, a receiver is made to own a second IC card, the transmitter performs an encryption process using the encryption key information inside the first IC card, and the receiver performs a decryption process using the encryption key information inside the second IC card. Obviously, by preparing a large number of IC cards, each storing the same encryption key information, and distributing these IC cards to members belonging to a predetermined group, encrypted communication is made possible among the members belonging to the group. Such a system, with which encrypted communication is performed among a large number of members using a plurality of IC cards, has already been proposed.
An encrypted communication system, to start with, is a system, with which encrypted data are transacted in a manner such that deciphering is possible only among members belonging to a predetermined group, and is premised on measures being taken to prevent the deciphering of encrypted data by outsiders not belonging to the group. However, in many cases in a company of comparatively large scale, etc., members belonging to a specific group change frequently.
For example, suppose that a project team, consisting of specific members, is formed in a company to carry out a specific project. In such a case, a situation may arise in which it is desirable for encrypted communication to be performed among just the members of the project team. Obviously in such a case, conventional encrypted communication arts enable encrypted communication of a mode such that deciphering cannot be performed by company staff not involved in the specific project. However, a project team in a company is liable to undergo a change of members frequently through dissolution, joining of groups, addition of new members, leaving of existing members, etc. Thus for practical use, an encrypted communication system that can flexibly accommodate changes of members is required. However, flexibility and security are generally conflicting elements and if flexibility is provided, security is inevitably compromised accordingly.
Thus an object of the present invention is to provide an encrypted communication system and an encrypted communication method that, when encrypted communication is performed among members belonging to a predetermined group, can accommodate flexibly even for changes of members and can yet ensure sufficient security. Another object of the present invention is to provide an IC card that can be used in such an encrypted communication system and encrypted communication method.